1. Field of the Invention
This invention relates to acoustic pulse-echo distance measuring apparatus, of the type in which a pulse of acoustic energy is transmitted towards a reflective surface whose distance is to be measured, and a reflected return echo of the pulse is received and detected, the elapsed time between the transmission of the pulse or shot and the reception of the echo being a measure of the distance of the reflective surface.
2. Review of the Art
In practice, the transmitted shot, which consists of a number of cycles of sound at a specific frequency, has a finite rise time since the transducer used to generate the acoustic energy may typically require several cycles to reach its full amplitude. This in turn means that the echo also must have a finite rise time. The transmitted shot will also have a finite fall or "ring-down" time as the transducer radiates stored energy following the end of the energization pulse used to generate the shot.
Traditional echo detection systems depend upon the magnitude of the echo exceeding some pre-determined "detection threshold", because the rising edge of the echo has each succeeding cycle slightly greater than the one which preceded it. It is difficult if not impossible to arrange that the echo will always exceed the threshold on the same cycle for every echo received. This is especially true at close target range where it is necessary that the sensitivity of the receiver be reduced in order that the transducer ring-down will not be detected.
The receiver sensitivity must change very rapidly to match the transducer ring-down characteristic, while any automatic gain control which might attempt to regulate the echo size must necessarily be of much slower response. This causes the echo as seen at the receiver's output to be reduced in size as the target moves into the range where the receiver's sensitivity is reduced to ignore the transducer ring-down.
If the echo size is reduced by even a small amount the detection threshold may not be reached on, for example, the fourth cycle of the echo rise but may instead occur during the fifth cycle thereby introducing a range detection error equivalent to the wavelength of the transmitted signal. In an ultrasonic distance measuring systems operating in air at 50 khz the error amounts to nearly a quarter of an inch.
In practice that echo size and shape in ultrasonic distance measuring systems is also somewhat variable because of such factors as air turbulence, sloshing liquid surfaces, the nature of the surface itself, dusty atmospheres, and so on.
In an article "DONAR: a computer processing system to extend ultrasonic pulse-echo testing" by Lees et al, Ultrasonics, July 1973, a system is described in which single cycle pulses are transmitted at a rapid repetition rate towards a target, and the peaks of the return echoes are detected by a digital sampling system. Such a system avoids any ambiguity as to the echo position, but is only suitable when a single cycle pulse is transmitted which is not usually practicable in industrial process control applications in which such a pulse would have insufficient energy, and the received signal would be too difficult to detect against a noisy background. A somewhat similar technique, in which the transmitted pulse is limited to a single half cycle, is disclosed as applied to a level measuring system in U.S. Pat. No. 4,850,226, issued to Allen et al. This approach severely limits the energy that the pulse can contain, as well as transducer efficiency, which restricts the range and noise immunity of the system.
Published European Patent Application No. 156636 (Salubre Investments Limited) discloses the extraction of a leading edge portion of a return echo and its analysis to provide data as to the nature of the reflecting surface.
U.S. Pat. No. 4,000,650 (Snyder) discloses the sampling of and digitising of return signals, the largest digital value received being utilized to determine the position of the return echo. The echo signals are however detected and isolated prior to digitisation, and the sampling rate is low compared to the frequency utilised. The system is therefore only useful for selecting between alternative echoes since its resolution is not high enough to be useful in determining with precision the position of an echo.
In U.S. Pat. No. 4,596,144, of which I am co-inventor, I employed threshold detection techniques for determining echo position, and similar techniques are utilized in the system described in the later U.S. patents of the co-inventor of U.S. Pat. No. 4,596,144, Steven Woodward.